Currently existing vascular sheaths were initially designed to safely and easily introduce angiographic catheters of relatively small diameters, e.g., 4-7 French (1.3-2.3 mm) percutaneously into arteries and veins. The advent of angioplasty balloon catheters led to the development of sheaths having inner diameters as large as 8-9 French (2.6-3.0 mm) to accommodate the early large balloon catheters. The basic design of the hemostatic valve typically provided at the proximal end of the sheath has changed only minimally during this time, the modifications being in materials and minor changes to improve hemostasis.
As familiar to those of skill in the art, the introduction of vascular stents (Palmaz, Wallstent, Strecker, Gianturco), percutaneously inserted vena cava filters (Titanium Greenfield, Bird's Nest, Simon Nitinol LGM Venatech), and atherectomy catheters (Simpson, T. E. C.) stimulated the development of vascular sheaths ranging in inner diameter from 10 to 14 French (3.3 to 4.6 mm). These larger sheaths have been designed either with the same hemostatic valves as the original smaller sheaths, or without a valve at all in the case of certain filter sheaths, since these latter sheaths are placed in the lower pressure venous system.
It has become apparent that passing stents and atherectomy catheters through the original-design hemostatic valve into the arterial system requires a stiff cylindrical sleeve to open the valve and prevent damage to the stent or atherectomy catheter as it passes through the valve. This is both cumbersome and results in a momentary massive blood leak under arterial pressure, as hemostasis is lost because the valve is opened by the sleeve.
U.S. Pat. No. 4,580,573 to Quinn and U.S. Pat. No. 4,540,411 to Bodicky both disclose catheter introducers having a single elastic valve. Such valves are designed to press tightly against a catheter as it is inserted and thereby attempt to prevent blood loss. Such valves have been found, however, to be inadequate for introducing stents and filters, which would be damaged by such pressure. On the other hand, if such valves are left open as devices are passed through them, a significant loss of blood would occur. U.S. Pat. No. 5,0009,391 to Steigerwald discloses a valve assembly for introducing catheters having two elastic disc valves. The discs are normally closed, but permit a catheter to pass through them while pressing tightly against the catheter. However, as was the case with other elastic valves described above, these designs are inadequate for introducing stents and filters since these devices would be damaged by such pressure.
A need therefore remains for a vascular sheath for introducing interventional devices into the vascular system, without pressing tightly against the interventional device as it is inserted while preventing a significant loss of blood.